The book is in Beta, but there are chapters for 5 databases, so there plenty of material already.
The idea of covering 7 databases, even superficially, is a good one, perhaps even better than for languages: while the choice of a language is often beyond the control of any single developer, the choice of a database (perhaps from the NoSQL family as a complement to a core, mandated SQL one) is possible. As with languages, trying to use a new database is if it was similar to one you’re already familiar with is likely to end badly. A good understanding of what each of them is good for, what problem they intend to solve, and what niche they fit better is mandatory.
I have been working with relational databases mostly so far (and, professionally, with one of the worst databases that people can charge money for), so I am eager to be introduced to the other kinds, without the usual cheerleading that is found in many tutorials.
In the coming weeks, assuming the book is timely updated, I’ll be blogging my way through it, just as I did through Seven Languages in Seven Weeks.